1. Field of the Invention
The present invention relates to compensation of Doppler shift in a mobile communication station.
2. Description of the Related Art
In a mobile communication system, signals which are transmitted from mobile stations moving relative to a base station are subject to the well-known Doppler effect, which causes a frequency shift in the frequency received at the base station relative to that which was transmitted by the mobile station. This frequency shift is referred to herein as the Doppler shift. The Doppler shift is dependent upon the speed and direction of movement of the mobile station relative to the base station. Thus, the Doppler effect can provide an increase or a decrease in the frequency, depending on the direction of movement of the mobile station relative to the base station. The magnitude of the Doppler shift is dependent on the speed with which the mobile station is moving relative to the base station.
Existing mobile communication installations provide a form of Doppler compensation, in that the frequency detection circuitry within the base station which selects a particular signal on a particular channel can take into account a certain amount of Doppler shift in the signal.
According to these known techniques, the Doppler shift is estimated from samples of the received signal. The estimate for Doppler shift is thus dependent on channel quality, and typically gets worse when the channel quality is poor and better as the channel quality improves. When there is no Doppler shift and the channel conditions are near the sensitivity level of a receiver, the application of a Doppler compensation algorithm degrades the performance of the receiver.
On the other hand, if there is a Doppler shift in poor channel conditions near the sensitivity level of the receiver, the receiver is not able to meet the reference sensitivity limits if a Doppler compensation algorithm is implemented.
According to one aspect of the present invention there is provided a method for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, the method comprising:
detecting the quality of the received signal; and
implementing a Doppler shift compensation in dependence on the detected signal quality.
The step of detecting signal quality can include estimating the noise energy component of the signal. This can either be used itself to generate a Doppler correction modification factor for controlling the Doppler shift compensation in dependence on the detected signal quality, or to generate a signal to noise ratio for the received signal, which would then be used to generate the Doppler correction modification factor.
The Doppler compensation can be implemented as any appropriate user defined function of signal quality. For example, it could be a linear function or a step function.
According to the present invention, the above described problem is overcome, because Doppler shift compensation is used only or mainly in good enough channel conditions. This provides an increase in performance for the receiver. Thus, the technique is provided for ensuring that Doppler compensation does not degrade the sensitivity of the receiver.
In the described embodiment, the adaptive method for Doppler correction applied to a GSM system first estimates the quality of the channel and uses the resulting modification factor to scale a calculated phase error between a reference signal and the actual received signal.
A particularly useful Doppler shift compensation technique for fast moving mobile stations is implemented by the following steps:
determining a channel impulse response for the channel on which the signal is received;
using the channel impulse response to estimate data bits of a selected portion of the received signal;
generating a reference vector using the channel impulse response and the estimated bits;
determining a Doppler characteristic using the reference vector and the selected portion of the received signal; and
using the Doppler characteristic to provide a Doppler shift compensation for the received signal.
According to another aspect of the present invention there is provided a system for compensating for Doppler shift in a signal transmitted between a mobile station and a base station in a mobile communication system, the system comprising:
circuitry for detecting the quality of the received signal; and
circuitry for implementing a Doppler shift compensation in dependence on the detected signal quality.